Locked-in oscillator circuits



Nov. 22, 1949 M. s. CORRINGTON 2,483,584

LOCKED-IN OSCILLATOR CIRCUITS Filed Dec. 8, 1.945 4 Sheets-Sheet 2 A TO NEY M. S. CORRINGTON LOCKED-IN OSCILLATOR CIRCUITS Nov. 22, "1949 4 Sheets-Sheet 3 Filed Dec. 8, 1943 mww INVENTOR (WP/PM. 0'. Cakewamm f' i RNEY Nov. 22, 1949 M. s. CORRINGTON 2,488,584

LOCKED-IN OSCILLATOR CIRCUITS Filed Dec. 8, 1945 4'SheetsSheet 4 QZ/ ER/ITOK.

INVENTOR fl/wPM/v I (ORR/N6 70M TTORNEY Patented Nov. 22, 1949 LOCKED-IN OSCILLATOR CIRCUITS Murlan S. Corrington, Camden, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 8, 1943, Serial No. 513,371

7 Claims.

My invention relates generally to improved locked-in oscillator circuits, and more particularly to novel methods of, and means for, extendin the lock-in range of frequency dividers of the locked-in oscillator type.

In his application Serial No. 430,588, filed February 12, 1942, patented August 22, 1944 as U. S. Patent No. 2,356,201, George L. Beers has disclosed and claimed novel circuits for receiving angle-modulated carrier waves employing a locked-in oscillator system acting as a frequency divider.

The locked-in oscillator operation may be explained on the following basis it being understood, of course, that the following explanation is my personal explanation: The angle modulated wave, of a predetermined mean or center frequency F, is applied to the input control grid of a tube of the pentagrid type. The output, or plate, electrode circuit is tuned to a desired subharmonic F/n, where n is a small whole number. In the illustrative embodiment described in the Beers patent the fifth subharmonic is developed across the plate circuit. The third grid of the pentagrid tube is regeneratively coupled to the plate circuit with the result that oscillations of mean frequency F/n are produced in the plate circuit. These oscillations occur even in the absence of signal voltage on the first grid. Suppose, now, that F=4300 kilocycles (kc) per second, and a signal of that mean frequency is applied to the first grid. Assume, further, that the plate circuit is tuned to 860 kc. In other words, n= and the oscillator section operates to provide subharmonic' oscillations of 860 kc. The grid voltage-plate current characteristic of the pentagrid tube is nonlinear; this would be true of any other type of tube. Hence, harmonics of F/ n are impressed on the oscillator grid. These harmonics which are present on the No. 3 grid include the fourth (n-DF/n (3440 kc.) and sixth (n+l)F/n (5160 kc.) harmonics of the oscillator current.

Hence, the signals'of 4300 kc. will beat with these fourth and sixth harmonics (niDF/n to provide a difference frequency the same as the desired fundamental frequency of 860 kc. (the fifth subharmonic of 4300 kc.). Now, this new component (which I will term the harmonic dif ference component to distinguish it from the normal oscillator current) will not, in general, have the same phase as the normal oscillator current. It, thus, is equivalent to injecting into the plate circuit an out-of-phase current. This causes the tube to act in the manner of the well-' known reactance tube. If the oscillator fre- 2 quency is not exactly one-fifth of the incoming signal frequency, the natural frequency of the plate circuit is pulled over until the oscillator frequency is exactly one-fifth. This causes the oscillator section to lock in with the applied signals.

The maximum amount the natural frequency of the oscillator can be pulled over, and still be locked-in, occurs when the harmonic difference current is 90 degrees out of phase with respect to the normal oscillator current, since this gives the maximum out-of-phase current. When the injected (harmonic difference) current is leading the normal oscillator current, the oscillator section frequency will be pulled to one side of its natural frequency (860 kc.), and when the injected current is lagging, the oscillator frequency will be pulled to the other side of its natural frequency. It will be recognized that there will thus be developed in the plate circuit a frequency modulated current locked-in with the applied signals.

There is a limitation on the lock-in range. This arises from the fact that the amplitude of the fourth and/or sixth harmonic on the oscillator third grid is normally quite small. Accordingly, the amplitude of the injected current is also small, and thus it follows that the lockin range of the oscillator section is limited. The result is that when the deviation of the oscillator frequency exceeds approximately 20 kc., from the oscillator mean frequency (860 kc.) the oscillator suddenly breaks out and its frequency returns to the mean frequency value. When the oscillator is used with a frequency discriminator in a frequency modulation receiver, distortion results, by virtue of this phenomenon. There are practical operating causes for the break out of the locked-in oscillator.

If the receiver is mistuned slightly, as is readily done in the case of frequency modulation receivers, the break out of the locked-in oscillator will occur on one end of the frequency swing, even for normal kc. swings or deviations. If the frequency modulation (FL/l hereinafter for brevity) transmitter over-modulates, the break out may occur on both ends of the swing. Likewise gradual drifts in frequency of the oscillator or discriminator can occur due to temperature or other changes during operation, thus causing mistuning and consequent break out.

It may, accordingly, be stated that it is one of the important objects of my invention to provide a eneral method of extending the lock-in range of a locked-in oscillator thereby to overcome the undesirable effects due to break on of the oscillator caused by insufficient harmonic on the oscillator grid.

Another important object of this invention is to provide a method of adjusting the range of a locked-in oscillator by control of the harmonic content'of the voltage on the oscillator grid or the input grid.

Another important object of my invention is to provide a highly practical method of increasing the lock-in range of a locked-in oscillator of the type shown in the aforesaid Beer's patent; the method comprising tuning the oscillator plate circuit sufficiently to one side of F/n so that it will readily lock in on one end of the frequency swing, and tuning the oscillator grid circuit sufficiently on the other side of the desired harmonic [(nl)F/n or (n+1)F/n] to help'lock in the oscillator on theopposite end of the swing.

Another object of my invention resides in the provision of a frequency-dividing, locked-in oscillator having its tank circuit tuned to the desired subharmonic frequency or the input mean irequency,"and also having an auxiliary circuit tuned to a predetermined harmonic of such subharmonic frequency; for extending the lock-in range of. the oscillator at the extreme frequency lue v he input ener y- A more specific object of this invention is to improve theoperation of an FM receiver of the type disclosed in the aforementioned Beers patent; the improvement residing in providing a frequency'divider of the locked-in oscillator type, normally operating the oscillator at a frequency different from the predetermined center frequency of, the following discriminator, when there is no incoming signal, and utilizing the res'ulting rectified outpu t;voltage for maintaining a tuning indicator in a no-signal indication state. Still; other objects, of my; invention are to improve generally the efficiency, reliability and range of. locked-in oscillators, andmore especially to provide locked-in oscillators'of extended range which are economical construction and assembly.

Other-features. and objects of my invention willbest be understood by reference to the following description, taken in connection with the drawing; inwhich I: have indicated diagram matically several; circuit organizations whereby my'invention may be carried into effect.

In' the'drawings:

Fig. 1 shows one embodiment of my invention,

Figs. 2 and 4; are vectorial analyses of my improved circuit,

I "Fig. 3. shows the effect. of the injected current on the tunedcircuit,

Fig. 5 graphically illustrates an advantage of the system,

Fig. 6 shows a modification,

Figs. 7a and. '7b7show; theresponse curves of the tuned circuits. of Fig. 6,

'Fig. 8 is a circuit; diagram of an FM receiver to which a further modification of. the locked-in oscillator is applied,

"Figs. 9a and 912 show the response curves of the tuned. circuits of the locked-in oscillator of Fig; 8,

Figs. 10, 11 and. 12 show respectively the offtune-'(or no-carrier), exact tune, and off-tune indications of the electronic indicator of Fig. 8,

Fig. 1.3 shows another embodimentof my invention,

Fig. 1.4 shows a method for amplifying the requiredharmonic.

frequency energy of a predetermined frequency F. The plate 4 has connected in circuit therewith'a resonant circuit 5 which is tuned to a subharmonic F/n of F, the symbol n denoting a small integer. By way of illustration, the fifth subharmonic may be employed. The plate 4 is, of course, established at a positive potential with respect to the grounded cathode. The second and fourth er dsioithetube are con ected in common to a source of positive potential, and these grids i nction. as a positive. sc ee rid for th ntermediate grid 5-.

The latter grid 6 is regeneratively coupled, as at 1, td'th'e plate circuit 5. The fifth grid of tube l is connected back to the cathode, and the grid therefore fu c i ns/ s. a pres o grid. The cathode ,3, grid 6 and plate 4 provide the oscillator section of the circuit. This oscillator section produces, oscillations of the, subharmonic frequency F/n. These oscillations are continu ously' produced, even in the. absence, of input ener y at g id. 2;. The osc atio d e o e across cir uit tare t ans e r d mus the 0 pling: condenser. L to. a y u izat on n w As shown (neglecting for a moment resonant circuit 8, ill, the cpil. ,1, has its lower end connected. to! round through the. res r n e latter. is b rnassed. for; high. frequ v ur e by the. condenser. L0. This s. s bs a a y similar to the locked-in oscillator circuit shown in the aforesaid Beers patent. The current from the tube not; sinusoidal, but. comes through as a'seri s. of. pu ses. These. aus ha n s th subharmq ic frequency 7/ to. be d op in the plate circuit 5. These harmonics will be app ied to. the rid 6. becaus th upl Eurthennore, the grid 6 is operating'with selfbias, and draws grid current during the positive swings of voltage. These pulses, also, contain he o r ha m nic f E/ Let us assume nQW, that}? hasa value of 4300 kg, Thisjfrequency, will beat; with the fourth (34%) kc.) and! Q sixth (5.1.6Qkc.) harmonics of h .resona ti equencyof hep ateq m therebxtorrofldet des red sub arm of 860 k In t ase it. is. assumed that; t e. t e 1t equal to 5; Whenthcfrcqucncyof theincoming i na s e a t vfive. t mest e at a fr uen f. theos llator; thaharm nic difi re c mponen wi be n; nhase. wi e. current in the oscillating plate circuit. The circuit becomes stable in ,this condition, and, the inj ected I current wi l oc n. eflncomine; 4.300 cs al w th the .6 urr nt nthe plat r t. Since the injected current, has the same phase and. freguency as, thenormal; current itis equivalent to anincreased.outputtfrom the tube.

Now.assume.mathem m t that h incoming requency i hi er. hen. .39 l d is i he. o lsr n ance. ut ha h i a has yet kediinr Thee icct i iou thha will. be. to ni qt ur ent of. s ght y, greater. ren enc han. 86.0 i kc. mm. he. an rc aFi 2.. 9A i el ct .assnmeshtohe atatins,

860,000 times per second, and represents the normal current in the oscillator tank circuit. Let AB be a-vector representing instantaneous injected current of frequency slightly greater than 860 kc. resulting from the fourth harmonic of the oscillator Voltage beating with the incoming signal voltage. This vector will rotate slightly faster than 860,000 times per second, and thus will have an angular velocity relative to OA equal to the difference of the two angular velocities.

Now consider further the instantaneous condition shown in Fig. 2. The injected current AB has a component AC shown dotted in phase with CA and another component AD (shown dotted) 90 out of phase with respect to 0A. The vector OB represents the resultant of OA and AB. Let this current OB be applied to a tuned circuit LC as shown in Fig. 3. Since the circuit LC is tuned to 860 kc., it will be at resonance with respect to the current 00, which is also 860 kc., and the total current C equals ia-l-ib. The quadrature current AD is aleading current at the instant shown, and the result is the same as though an additional condenser C is in the circuit LC. The effect was to decrease the natural frequency of the tuned circuit LC by an amount proportional to the magnitude of the vector AD.

Now consider the condition at a later instant, as shown by Fig. 4, the incoming frequency being the same as in the preceding discussion of Fig. 2. The vector AB has rotated to the new position as shown. The injected current AB now has an in-phase component AC as before, but the cornponent AD is now lagging instead of leading. If the current OB is now impressed on the circuit of Fig. 3, the lagging component AD will cancel part of the leading current through capacity C, and this will be equivalent to reducing the capacity C since it is now drawing a smaller leading current. This will raise the resonant frequency of the tuned circuit LC by an amount proportional to the magnitude of the vector AD.

It is now evident that the circuit of Fig. 1 behaves like a reactance tube circuit. It is easy to see that if the frequency of the incoming signal is approximately five times that of the tuned circuit 5, a point will be reached when the frequency of the tuned circuit becomes exactly onefifthof the incoming signal frequency. When this happens the oscillator will lock-in" with the incoming signal. This means that the amplitude and phase of the plate current now remain fixed with respect to the incoming signal.

If the incoming signal is exactly five times the frequency of the tuned plate circuit, the vector AB will be in phase with CA, there will be no lagging or leading component, and there will be no tendency to change the resonant frequency of tuned circuit 5. As the incoming signal frequency is decreased, the oscillator will remain locked in with it because the vector AB rotates to some position such as that shown in Fig. 2, thus decreasing the oscillator frequency. A further decrease in frequency will rotate the vector until it is 90 out of phase with respect to 0A. Since this position gives the maximum amount of quadrature current it corresponds to the maximum amount that the oscillator frequency can be pulled over, and thus gives the lower limit of the lock-in range. If the incoming signal frequency becomes greater than five times the plate circuit frequency, the conditions will be similar except that the vector AB will be lagging, as shown by Fig. 4, instead of leading. The upper limit of the lock-in range is reached when the 6 injected current lags by i The lagging current tends to reduce the effective capacity of the circuit, and thus raises the frequency. f

. The amount ,of fourth and/or sixth harmonic current which is applied 'to' grid 6 is normally relatively small. This limits the lock-in range of the oscillator, since it limits the amount of the injected current. The result is that when the deviation of the incoming signal exceeds approximately kc., the oscillator suddenly breaks out and the frequency goes back towards center. This will cause distortion. Reference is made to Fig. 5 wherein there is shown the break-out gap produced in the output from a frequency discriminator. The practical operating disadvantage of this break-out effect resides in the factthat if a receiver is mistuned slightly, the break-out will occur on one end of the frequency swing and even for normal 75 kc. swings. Such mistuning 'of a receiver is quite possible, since the average operator of a frequency modulation receiver finds it diificult to tune the receiver exactly. Again, if the FM transmitter station overmodulates, the break-out will occur on both ends of the frequency swing.

In accordance with my invention, the breakout effect is substantially eliminated by extending the lock-in range of the oscillator. This is very simply accomplished by increasing the amplitude of the harmonics on grid 6 at the ends of the lock-in range where they are needed most. One simple way of accomplishing this result is to insert in series with the grid coil 1' an auxiliary resonant network consisting of coil 8 and condenser ll arranged in parallel. The parallel resonant circuit 8--l I is tuned to either the fourth or sixth harmonic of the subharmonic F/n, when n=5. Preferably, the fourth harmonic is employed. As a general proposition the auxiliary circuit 8Il may be tuned to nearly any harmonic of the subharmonic frequency, and will give improved results. If circuit 8Il is tuned to the second harmonic. the tube will actually distort this harmonic thereby yielding added fourth and sixth harmonics. When the circuit 8-ll is tuned to the third harmonic of F/n, then the result will be that the sixth harmonic will also be built up. Hence, it should be understood thatthe fourth harmonic is shown merely by way of illustration. By way of specific illustration the circuit 5 may have an inductive mag nitude of 0.762 millihenry and a capacity of 45 micro-microfarads (mmf.). Coil 8 may have a value of 40.8 microhenries while condenser I I can have a value of 52.5 mmf.

The effect of the auxiliary tuned circuit 8--ll is to increase the magnitude of the vector AB representing the injected current, as shown in Figs. 2 and 4. The resultant increase in the magnitude of the vector AD increases the lock-in range of the oscillator over the case where the auxiliary circuit 8--|l is omitted because the effect of this component on the resonant frequency of tuned circuit 5 is increased proportionately. The dotted crests of the curve in Fig. 5 show the effect of the auxiliary grid circuit 8-H insofar as overcoming the break-out is concerned. When the oscillator frequency reaches the end of its range, instead of dropping back suddenly to center frequency, itshifts to another frequency etc. This has the effect on wide deviations of;

magma producing the dotted substantially fiattop crests shown in Fig. This is amuch less objection}- able form of distortion. It is scarcely audible for deviations greater than 150 kc. The break-out? effect will not occur until the applied energy at grid 2 is reduced to a small value by the selectivity 'of circuits which may precede grid 2. V I I It will be appreciated that by inserting intothe circuit of grid 6, that is the oscillator section grid, a tuned circuit which is resonated to the fourth harmonic of the subharmonic F/n there is seicured a very great increase in the magnitude of the fourth harmonic voltage. Of course, by increasing the Q of the auxiliary circuit 8-1 I, the fourth harmonic com'pone'nt on grid Ii maybe still further increased. However, there exists the dis advantage that the sharpness of the auxiliary circult may be increased to a point such that the fourth harmonic component will not be built up at the ends of the swing where it is most needed; whereas if the Q of theauxiliary circuitis made low enough to broaden out thecircuit response, a reduction in output of the desired harmonic results. I g e I have found that this particular disadvantage may be effectively overcome in I several ways. One of these ways is shown in Fig. 6. Here the single auxiliary circuit I I'I l is replaced by an overcoupled double-tuned circuit. The numerals II and I I designate respectively the overcoupled double-tuned circuits; i. e. circuits whose coupling is tighter than critical coupling, The coupling transformer I2 of these overcoupled circuits has its constants chosen 'so that the response curve of the overcoupled circuits is indicated by the double-peaked curve P in Fig. 712. It will be noted from the curve P that one ofthe peaks of the re sponse curve occurs ata frequency less than the fourth harmonic frequency while the second peak occurs at a frequency greater than The oscillator section plate circuit 5 in that case has its natural frequency located midway between the spaced peak frequencies of the circuits I I and Fig. 7b shows the oscillator plate circuit rescnant frequency in relation to the curve P of Fig. 7a. By this method the oscillator plate circuit is tuned to the mean or center frequency F/n, whereas the auxiliary network has its spaced peak frequencies located on respectively opposite sides of the desired fourth harmonic frequency. If 'de sired, the o'vercoupled tuned circuits could be replaced by a plurality of tuned circuits whose con- M stants are so related as to extend the lock-in range of the oscillator section.

In Fig. 8 I have shown still another method of extending the lock-in range of the oscillator sec tion. The system shown in Fig. 8 includes the locked-in oscillator acting as a frequency dividing network in a receiving system of the type disclosed in the aforementioned Beers patent. As more fully explained in the Beers patent the locked-in oscillator functions concurrently to reduce or divide the mean frequency of the applied FM signal waves, and proportionately to reduce the extent of frequency deviation of the waves. Assuming that the FM receiver is of the superheterodyne type and that the networks prior to Value.

the 'lccked-in oscillator tube arecon'ventional in nature, the transformer I3 will haveits primary circuit I4 tuned to the operating intermediate frequency (I. F.) of the system.

As fully explained in the Beers patent, the re ceived FM waves are those which are transmitted in the assigned FM band of 42-50 megacycles (me). Of course, the invention is not limited to any particular frequency band or to the reception of FM waves, since it is generally applicable to angle modulated carrier waves. Those skilled in the art are fully aware of the fact that the FM waves transmitted in the aforementioned assigned FMband are now allotted a maximum over-all frequency swing of kc. with respect to the mean or center frequency Fe. The extent of fre quency deviation is dependent upon the amplitude of the modulation signals at the transmitter, while the rate of frequency deviation is dependent upon the modulation frequency per 'se.

The collected FM waves are selected in one or more stages of tunable radio frequency amplification, after which they are combined with 10'- 'cally-produced oscillations at the first detector network. The output of the first detector or converter is the I. F. (intermediate frequency) energy. In other words, the I. F. energy is the FM Wave whose mean frequency has been reduced to a much lower frequency, but whose frequency deviation is unchanged. After amplification in one or more stages of I. F. amplifiers, the I. F. energy is applied to the locked-in oscillator for concurrent frequency division and frequency deviation reduction. I

Above the I F. transformer I3 I have depicted, in a purely graphic manner, the ideal response curve of the coupled primary and secondary tuned circuits I 4 and. I5. It will be seen that the mean frequency is located at 4.3 mc., Whereas the pass band of the network is substantially 150 kc. wide. This signifies that the network I4-I5 is capable of efficiently transmitting the entire frequency swings of the FM wave whose mean frequency has been reduced to the operating I. F.

While the value of 4.3 me. has been ass'igne'd as the operating I. F. value, it is to be clearly understood that any other satisfactory value may be employed depending upon the var ious factors met with in the design of the receiver.

since the locked-in oscillator circuits in Fig. 8 are exactly the same as shown in Fig. 1, the same reference numerals are employed. The FM signal energy with its mean frequency at the operatmg I. F. value is applied to the input grid 2. The

- input grid 2 is connected to the high alternating potential Side of the secondary circuit I5. The low potential side of circuit I5 is returned to the rounded cathode 3 through a resistor and condenser network designated by the numeral IS. The function of the network I 6 is to provide voltage a'cr'o'ssthe resistor element in response to grid current flow through the input grid circuit. Such grid voltage developed across the network I6 may be utilized for automatic volume control (AVC). The AVC voltage is employed automatically to bias the control grids of preceding amplifier tubes in a manner well-known to those skilled in the art.

The plate 'or primary circuit 5 in the embodiment of the invention shown in Fig. 8 is resonated to a frequency located to one side of the mean output frequency. For example, if the mean output frequency is to be 860 kc-., assuming a frequency division by a factor 5, then circuit 5 is tuned to a frequency greater than 860 kc. By way of spacific example, the'plate circuit frequency could be 8'75 kc. The auxiliary resonant circuit 8l l, which is arranged in circuit with grid 6 and the secondary or feedback coil 1', is tuned to a frequency less than the fourth harmonic of 860 kc. In other words, circuit 8l l is tuned to less than 34 kc. For example, the frequency could be 3380 kc. Of course, it could also be tuned to less than the sixth harmonic 5160 kcL were it desired to employ the sixth harmonic. As a specific example of constants for circuit 5, there may be employed an inductive value of 0.735 millihenry and a capacity value of 45 mmf. at the frequency of 875 kc. Coil 8 and condenser II could be 40.8 microhenries and 54.4 mmf. respectively at 3380 kc.

In Figs. 9a and 9b I have depicted in a purely qualitative manner the relation between the response or resonance curves of the oscillator plate circuit 5 and the auxiliary tuned circuit 8l l It will be noted that the peak response of the plate circuit 5 occurs at greater than 860 kc., that is, the selected subharmonic frequency lies on the low frequency portion of the curve, while the resonant frequency of the auxiliary tuned circuit is less than 3440 kc., that is, the harmonic of the selected subharmonic lies on the high frequency portion of the curve. The important advantage of this off-tuning of the circuits 5 and 8H is that the oscillator section will lock in on one end of the frequency swing by virtue of the mistuning of circuit 5, while it will be locked in at the opposite end of the swing by virtue of the circuit 8l I. In other words, the lock-in range of the oscillator section will be extended at the extremes of the range.

Above the plate circuit 5 in Fig. 8 I have graphically represented the ideal response curve of the oscillator section output network. It will be noted that it has a pass band width of 30 kc., while the mean frequency has a value of 860 kc. This is appropriate since the effect of the locked-in oscillator network has been to divide the mean frequency of the FM wave energy by a factor of 5, and the over-all frequency deviation range has also been divided by the same factor. Additionally, there is secured a substantial reduction of amplitude modulation effects which may have been created on the FM wave energy in the transmission through space or during the passage of the signal energy through the receiver networks.

This elimination of amplitude modulation effects is secured, as explained in the aforesaid Beers patent, without the use of any special amplitude limiter stage. The advantages of frequency division at this point of the receiving system also have been explained in the Beers patent. The extension of the lock-in range of the oscillator accomplished by this invention as compared with the arrangements shown in the Beers patent will enable reception of waves which are frequency modulated over a wider range and will guard against the distortion which might otherwise occur by reason of the aforementioned break-out" effect. Since the FM wave energy developed across the plate circuit 5 is modulated in strict accordance with the originally received FM waves, save that the mean frequency and extent of frequency deviation have been proportionately reduced, the output of the locked-in oscillator may now be subjected to any wellknown form of discriminationand rectification.

The numeral [6 denotes a discriminator input fiers l1 and [8. The discriminator input circuit I6 is shown only in block diagram, because it is not part of the present invention so far as its specific construction is concerned. The type of discriminator input circuit shown in the Beers patent may be utilized. That type of discriminator input circuit is disclosed and claimed by J. D. Reid in application Serial No. 353,028, filed Aug. 1'7, 1940, U. S. Patent No. 2,341,240, granted February 8, 1944. In place of the Reid discriminator there may be employed the well-known form of discriminator shown by S. W. Seeley in his U. S. Patent No. 2,121,103, granted June 21, 1938. There may, also, be employed for the discriminator input circuit I6 the form wherein each rectifier has an input circuit individual to itself, and the input circuits are oppositely mistuned relative to the mean frequency of the applied signal energy as in the U. S. patent to Conrad, 2,057,640. In any event, and regardless of the construction of the discriminator input circuit Hi, the opposed diodes I"! and I8 will be separately fed with signal energy which has the form of amplitude modulated wave energy by virtue of the action ofthe discriminator input circuit which translates the FM wave energyinto corresponding amplitude modulated carrier wave energy.

As will be noted from Fig. 8, the output load consists of a pair of series-connected resistors I9 frequency Fc the resultant output voltage of thedetector is zero. The maximum output occurs at the opposite peak limiting frequencies F1 and F2. Those skilled in the art of radio communication and FM transmission and reception are well acquainted with the manner in which this device operates.

Each of the opposed rectifiers includes in circuit therewith its respective load resistor. At the instant when the applied signal energy has the mean frequency value, the rectified voltages across the resistors l9 and 20 are equal in magnitude and opposite in polarity. Hence, the resultant potential at the upper end of resistor I9 is zero. On the other hand the potential at the upper end of resistor l9 will, vary in magnitude and polarity, depending upon the instantaneous extent and direction of frequency deviation with respect to the mean frequency. Accordingly, the Voltage taken off from the output load of the opposed rectifiers corresponds to the original modulation signal voltage applied to the carrier at the FM transmitter;

Since the plate circuit 5 of the oscillator section of tube l in Fig. 8 is normally mistuned with respect to the mean frequency of 860 kc., advantage is taken of that fact to provide a novel, highly effective and simple form of resonance indication. In providing a resonance indicator for the receiving system shown in Fig. 8 there is utilized an indicator tube 22 of the well-known 6E5 type. Those skilled in the art are well acquainted with this type of indicator tube. The numeral 22 denotes the indicator tube. It comprises a pair of electronic sections. One of these sections is a triode consisting of a cathode electrode 23, acontrol grid 24 and a plate 25. The

agent-s4 ll cathode 23 is. connected to ground by means of a resistor S which is adjustable in magnitude. The control grid. 24 is connected to the ungrounded end of resistor I9 through a pathwhich consists of resistor'R andthe filter resistor 28.

The grid 24 is connected to ground by a condenser C for bypassing audio frequency voltages. The resistor 28 and the pair of. parallel condensers 29 provide a de-emphasis network whose function is well known in the art of frequency modulation reception. Briefly, the de-emphasis network acts to diminish the response at the higher audio frequencies, since during transmission of the FM waves such higher audio frequency components may have been disproportionately emphasized. It should be noted that the lead to resistor R can be connected to either side of resistor 28. It will, therefore, be seen that the control grid 24 f the triode section of the indicator tube is varied in potential in accordance with the magnitude and polarity of the voltage existing at the upper end of resistor l9. Since the condenser C filtersv out any audio frequency variations, it follows that the voltage of grid 24 will be a function of, the variations in mean frequency of the FM wave energy applied to the discriminator input circuit IS.

The upper electronic section of tube 22 is the indicator section. It consists of a cathode section 23' which is actually an extension of the cathode electrode 23, except that the sections 23 and 23' are spaced by a non-emissive section. However, the sections 23' and 23' are at: a common potential. The indication electrode has a configuration of. an inverted frustum of a cone. The numeral 26. denotes the outwardly flaring electrode. The inner face of electrode 26 is coated with a fluorescent composition which will fluoresce. upon the impingement of electrons emitted by the cathode section 23. In. other words, the electrode 26 functions asa fluorescent target.

The target electrode isconnected to a sourceof +250 volts, and the triode'plate 25. is con nected to the same potential point through the resistor r. The numeral 21 designates. an electron deflection rod or electrode whichis directly connected to the plate 25'. It will, therefore, be appreciated that the rod 21. follows the potential. variations of plate 25. More specific details of the construction of an indicator. tube of this type will be found disclosed and claimedby H. M. Wagner in his U. S. Patent No. 2,051,189, granted Aug. 18, 1936.

It is sufficient for the purposes of this application to explain that the magnitude of the cathode resistor S'is adjusted so that grid 24 has a normal direct current voltagewith respect to cathode 23 such that the electron shadow provided on the target 26 is of minimum width. Let. itbe assumed that no signal energy is bein re.- ceived. Under these conditions the indicator tube will function in the following manner: The normal voltage drop across the cathode resistor S gives the grid 24 a normal negative bias such as to provide a normal plate current flow through the resistor r. The deflection electrode 21 is biased by the voltage drop across the plate resistor r so as-to provide a minimum deflection of electrons. Since, in theabsence of signal energy, the plate circuit of the oscillator'tube I is tuned-to a frequency greater than 860 kc., and since 860 kc. is themean frequency of the FM detector characteristic, there will be developed a certain positive potential at the upper end of' resistor [9 which is applied togrld- 24. Thispositive voltage renders the grid 24 less negative with respect to the cathode, and permitsan-increased space current flow through the plate resister 1'. As a consequence, the rod 21 will become effectively more negative with respect to the target 26. This means that the electrons will not be curved around the rod, and will not strike the target area in front of rod 21. In other words, the shadow sector will widen.

In Fig. 10 I have shown the appearance of the. shadow area 26 on the fluorescent face of target 25 when no signal energy is being received. Fig. 11 shows the minimum width of the shadow area 26 when the signal energy applied to the FM. detector has a mean frequency of exactly 86% kc. It will be noted that the shadow area has been narrowed to a considerable extent, because. the voltage at the upper end of resistor 59 is zero when the applied signal energy has a mean frequency at the discriminator input circuit which is equal to the center frequency of the FM detector characteristic. Hence, the deflection electrode 2'5 will be less negative with respect to the target 28, and the. electrons will be curved around the rod to strike the target behind the rod and thus close the eye.

Fig. 12 shows the appearance of the target 26 when the signal energy applied to the discriminator input circuit has a mean frequency which is less than 860 kc. The result is a very bright overlap instead of an electron shadow. Fig. 10 also shows the appearance of the target when the signal energy applied to the discriminator has a mean frequency which is greater than 860 kc.

It will now be appreciated that I. have provided a simple and effective resonance indicator which indicates by virtue of' minimum shadow area that the receiver is exactly in tune. However, when the mean frequency of the applied signal energy drifts away from Fe toward I": (that is, in the direction of the normal frequency of plate circuit 5 of the locked-in oscillator), the shadow area 26' widens out to indicate that the receiver is mistuned to that side ofexact tuning. On the other hand, when the mistuning occurs in the opposite direction, toward Fr, there appears an area of very bright overlap thereby instantly indicating that condition of mistuning. In the prior art, resonance indicators of the fluorescent target type used on standard FM receivers did not distinguish between absence of signal energy and mistuning; It will be seen that in accordance with my invention it is possible under some conditions to distinguish absence of signal energy from mistuning.

The methods of Figs. 1,.6v or 8ican also be used in the input grid circuit of tube I. Fig. 13 shows one embodiment of my invention wherein the tuned circuit 8-! l is located in the circuit of grid 2. I have found that this. also will build up the required harmonic onv the grid 2 and increase the lock-in range.

The invention is not restricted to frequency modulated signals, but can also be used at any fixed frequency. The increased lock-in range is desirable when operating at a fixed frequency to assure that the oscillator will-remain locked in even though the circuits may become slightly misaligned due to temperature changes,,voltage changes, etc. The use of a locked-in oscillator with a frequency discriminator represents only one application. It can be used to obtain secondary frequency standards from a given primary standard such as a crystal oscillator; it

angina" can be used to compare two frequencies; to opcrate electronic clocks; or in any other application where a definite ratio of frequencies is desired. It .should be further noted that my invention is not limited to the use of a pentagridtype tube- It will operate with any tube which is normally used as an oscillator.

Fig. 14 shows another embodiment of my invention. The capacitor 35'couples the tuned plate circuit to the grid 38 of an amplifier tube 38' which is biased to give high distortion. Generally, any non-linear amplifier may be employed as a harmonic generator. The tuned circuit 39 is tuned to the desired harmonic that is to be impressed on the oscillator grid. This tuned circuit '39 is coupled as at 31 to the grid 6 of tube I. This modification operates in the same manner as the circuit 8--H of Fig. 1, except that the required harmonic is stronger than that obtained in Fig. 1. It'can also be adjusted by varying the gain of tube 38, which although shown as a triode can be any other well known amplifier tube, such as a pentode. In like mannen'the amplifier tube can also be used in combination with any of the other circuits described above to control and increase the harmonic content in' the circuits.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In combination with a tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element high frequency current which is angle modulated and has a desired mean frequency, a resonant tank circuit reactively coupled to said oscillator grid and output electrode and tuned substantially to a desired subharmonic frequency of said mean frequency for normally and continuously producing oscillatory current of said subharmonic frequency, means including a selective network tuned substantially to a harmonic of said subharmonic which differs from the mean frequency by the subharmonic frequency, means coupling said selective network to said tank circuit and oscillator grid, means for deriving from the angle modulated current of subharmonic frequency the modulation thereof, a tuning indicator, and. means responsive to normal current of said tank circuit for maintaining the indicator in no-signal indication state.

2. In combination with a tube provided with at least an electron emitter, a control element, a grid and output electrode, means for applying to the control element frequency modulated carrier current of a desired frequency, a resonant tank circuit tuned to a subharmonic frequency of said desired frequency and including a first inductor connected to said output electrode and a second inductor connected to said grid electrode and coupled to said first inductor so as to produce continuously oscillations of said subharmonic frequency, a selective network auxiliary to said tank circuit and in circuit with the tank circuit and tuned to a harmonic of said subharmonic which differs from the carrier frequency by the subharmonic frequency, and additional means for applying harmonic current developed in said selective network to said oscillator grid.

3. In combination with a locked-in oscillator tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the controlelementhighfrequency current which is angle electrode and being tuned normally to a frequency which is greater than a desired subharmonic frequency of said mean frequency so that said subharmonic lies on the low frequency portion of the response'curve of said tank circuit, a selective network tuned to a frequency which is less than a desired harmonic of said subharmonic so that said harmonic lies on the high frequency portion of the response curve of said network, which said harmonic differs from the means frequency by the subharmonic frequency, said selective network being coupled to said tank circuit'and to said os-. cillator grid, means for rectifying the current of subharmonic frequency in said tank circuit, a tuning indicator, and means responsive to rectified current in no-signal state for controlling the indicator.

4. In combination with a tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element frequency modulated high frequency current of a desired center frequency, a resonant tank circuit reactively coupling said oscillator grid and output electrode and tuned substantially to a desired subharmonic frequency of the said center frequency, a pair of coupled selective networks each tuned to a harmonic of the subharmonic frequency, and means connecting said selective networks to said oscillator grid thereby applying harmonic current developed in said selective networks to said oscillator grid.

5. In combination, in a frequency modulation receiver, a tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element frequency modulated current of a. desired mean frequency, a resonant tank circuit reactively coupled to said oscillator grid and output electrode and tuned to a frequency which is greater than a desired subharmonic frequency of said mean frequency so that said subharmonic lies on the low frequency portion of the response curve of said tank circuit, a selective network tuned to a, frequency which is less than a desired harmonic of said subharmonic so that said harmonic lies on the high frequency portion of the response curve of said network, which said harmonic differs from the mean frequency by the subharmonic frequency network to said tank circuit and oscillator grid said selective network being in series with said coupling means and said oscillator grid, and means for deriving from the frequency modulated current of said tank circuit the modulation thereof.

6. In combination with a locked-in oscillator tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element high frequency current which is angle modulated and has a desired mean frequency, a resonant tank circuit reactively coupling said oscillator grid and output electrode and tuned normally to a frequency which is greater} than a desired subharmonic frequency of said? mean frequency so that said subharmoniclies on the low frequency portion of the responsescurve of said tank circuit, ,a resonant. circuittuned to a.

dividing, oscillator tube provided with at least.'

an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element frequency modulated current of a, desired center frequency, a resonant, tank circuit including primary and secondary windings regeneratively coupling said oscillator grid and output electrode, said primary windingbeing tuned to a frequency which is great-, er than a desired subharmonic frequency of said mean frequency so that said subharmonic lies on the. low frequency portion of the response curve of said tank circuit, a resonant circuit tuned to a frequency which is less than a desired harmonic of said subharmonic so that said harmonic lies on the high frequency portion of the response 16 ur f sai reson n ircuit. whi s a monic differs from the mean frequency by the subharmonic frequency, said last resonant cirs cuit being auxiliary to the regenerative coupling, said resonant circuit being in series with said secondary winding and said oscillator grid whereby harmonic current developed in said resonant circuit affects the electron stream.

MUBLAN S. CORRINGTQN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,933,970 Curtis Nov. '7, 1933 2,098,386 Hansell Nov. 9, 1937 2,149,721 Bell Mar. 7, 1939 2,245,134 Klaiber June 10, 1941 2,259,000 Nyquist Oct. 14, 1941 2,286,410v Harris, June 16, 1942 2,296,089 Crosby u Sept. 15, 1942 2,344,678 Crosby Mar. 21, 1944 2,356,201 Beers Aug. 22, 1944 2,440,653. Corrington Apr. 27, 1948 2,451,584 Stone Oct; 19, 1948 

